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Zr–2.5Nb alloy, with its high yield strength and low hydrogen absorption, is used in Pressurized Heavy Water Reactor (PHWR) as pressure tube. Hydrogen in solid solution form and in hydride precipitate form may alter the mechanical properties of the Zr–2.5Nb pressure tube. In the present study, the effect of hydrogen on thermal creep behaviour of Zr–2.5Nb alloy pressure tube material is characterised in the temperature range of 350–450 °C and a stress ranging from 0.7 to 0.9 times of yield strength. The variation in rupture time, minimum creep rate, stress exponent and activation energy is determined for as-received, 150 and 200 wppm hydrogen charged samples. An attempt has been made to understand the role of hydrogen in solid solution form and as hydride precipitate form on thermal creep behaviour of Zr–2.5Nb pressure tube alloy.

The majority of data reported in open literature on hydride embrittlement of Zr alloys is based on hydrogen not deuterium which evolves during aqueous corrosion of in-core components in pressurized heavy water reactor operations. It is assumed that hydrogen and deuterium have similar effects on tensile properties and fracture toughness of Zr alloys. The thermal and physical properties as well as stress free transformation strain of hydrides and deuterides are different. Because of these differences, hydrogen and deuterium may affect tensile and fracture properties of Zr alloys to a different extent. The hydrogen isotope effects on tensile and fracture properties were investigated using samples machined from Zr–2.5Nb pressure tube (PT) material charged with hydrogen between 30 and 200 wppm and tests were carried out between 25 and 300 °C. The tensile properties and fracture toughness of the hydrogen charged Zr–2.5Nb PT material were compared to the Zr–2.5Nb PT material charged with deuterium. The observed effect of hydrogen isotopes on tensile and fracture toughness was rationalized in terms of stress free transformation strain and the temperature dependence of the fracture toughness of the hydrided material was correlated with the presence of axial splits on fracture surfaces. The effect of hydrogen isotope on tensile and fracture properties was investigated for the Zr–2.5Nb pressure tube (PT) material charged with hydrogen between 30 and 200 wppm and in the temperature range of 25 and 300 °C and compared with the corresponding values determined for the Zr–2.5Nb PT material charged with deuterium. It was observed that for a given concentration up to 200 wppm, hydrogen and deuterium charged Zr–2.5Nb PT material have practically same tensile and fracture properties at all test temperatures between 25 and 300 °C indicating no effect of nature of hydrogen isotope on tensile and fracture properties of Zr–2.5Nb PT material.

In the present work, a modified 9Cr-1Mo steel is subjected to normalizing and tempering treatment with or without an intermediate rolling, which was carried out at 1050 °C in austenitic phase and 550 °C in metastable austenitic phase. The tempering was carried out at 700 °C and 750 °C to improve the strength by precipitation hardening and refining the microstructure. The ductile to brittle-transition temperature (DBTT) and tensile properties have been evaluated at 20 °C, 550 °C and 650 °C for the material subjected to various thermomechanical treatments. Rolling performed in austenitic phase showed improvement in the upper shelf energy and reduction in the DBTT compared with the material rolled in metastable austenitic phase. Rolling in metastable austenitic phase improved the yield strength at elevated temperature (39 pct at 20 °C, 31 pct at 550 °C and 91 pct at 650 °C) and was accompanied by a reduction in ductility (21 pct at 20 °C, 32 pct at 550 °C and 35 pct at 650 °C) compared with the as-received condition. The influence of grain size, low angle boundaries and precipitates on DBTT and high temperature strength has been studied using EBSD and TEM analyses. An increase in high temperature strength is found to increase with the area fraction of fine M23C6/MX precipitates, whereas low angle boundaries and effective grain size influenced the DBTT behavior. The combined effect of precipitation strengthening and strain hardening leads to improvement in the high temperature mechanical strength while maintaining adequate toughness. The effect of aging (for 72 hours at 650 °C) on heat-treated and -rolled samples showed a small decrease in yield strength at 20 °C. A decreasing trend of fracture toughness is observed with the increase in yield strength.

Mixed molybdates of cobaltand nickel with compositional formulae Co1-xNixMoO4 (where x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1.0) have been synthesized by microwave-assisted co-precipitation method and examined aselectrocatalysts for oxidation of methanol and water and as supercapacitors in 1.0 M KOH.Cyclic (CV) andlinear sweep (LSV) voltammetriesand chronoamperometry techniques were used in the investigation. Structural and surface characterizations of materials were performed by XRD,Raman, SEM, HRTEM andXPS.The study has shown that introduction of Ni for Co in the CoMoO4 matrix increases the catalytic activity towards methanol (164.5 mAcm−2) and water (90 mAcm−2) oxidations, the activity, however, being the greatest with 0.5 mol Ni. Also, the capacitance of this active electrode material (611 Fg-1) was found to be much superior to pure CoMoO4(140.6 Fg-1)or NiMoO4 (89.79 Fg-1).

Highly dispersed Pd-nanoparticles (NPs) are obtained on CoMoO4-doped graphene nano-sheets (GNS) by microwave-assisted polyol reduction method for use as electrocatalysts for methanol oxidation (MOR) and oxygen reduction (ORR) reactions. The activity of Pd/10 wt%CoMoO4-GNS towards MOR and ORR has been examined by cyclic (CV) and linear sweep (LSV) voltammetries and chronoamperometry measurements in 1 M KOH at 25 °C . The structural characterization of the hybrid catalyst was performed by X-ray diffraction, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy. Results have shown that the novel Pd/10 wt%CoMoO4-GNS electrode has the MOR and ORR activities superior to Pd/GNS.

In this study, indentations were made on Zr-2.5%Nb pressure tube material to induce multi-axial stress field. An I-shaped punch mark was indented on the Pressure tube material with predefined punch load. Later material was charged with 50 wppm of hydrogen. The samples near the punch mark were metallographically examined for hydrides orientation. It was observed that hydrides exhibited preferentially circumferential orientation far away from the indent to mixed orientation containing both circumferential and radial hydrides near the indent. This is probably as a result of stress field generated by indentation. Extent of radial hydride formation was observed to be varying with indentation load.

Steels for ship building applications has to possess adequate resistance to propagation of fatigue cracks, as majority of the failures in service are due to metal fatigue. In this work, the fatigue crack growth rate (FCGR) behaviour within the Paris regime of two high strength low alloy (HSLA) steels were studied. In particular the experiments were directed to reveal the effect of load ratio(R) (Tension-Tension) on the Paris law constants for the two grades of HSLA steels. Results indicated that there is an increase in Paris slope ‘m’ and decrease in Y intercept ‘C’ with increase in load ratio for both the steels. Fractography study was carried out using SEM at locations corresponding to various values of stress intensity factor in order to reveal possible reasons for acceleration of crack growth with change in R ratio from 0.1 to 0.5. It was found that secondary cracks are predominant in Steel A at a load ratio of R=0.1 as compared to R=0.5. However for Steel B, secondary cracks were found at both the load ratios. From the plot of crack growth ‘a’ vs no. of cycles ‘N’, it became evident, the number of cycles for the same range of crack length is lesser for R=0.1 than that for R=0.5 for both Steel A and Steel B. Limiting values of ‘∆K’ and ‘K’ max has been obtained for various crack growth rates ‘da/dN’ at different load ratio for steel A and steel B.

In the present work, effect of normalizing (950–1050 °C) and tempering (650–700 °C) has been investigated to study the microstructure and mechanical properties of modified 9Cr-1Mo steel (G91) at room temperature (RT) and 650 °C. To improve the high temperature mechanical properties, hot rolling has been done at optimized normalizing and tempering conditions. Microstructural observations show that an increase in tempering temperature leads to decrease in mechanical strength both at RT and 650 °C due to the growth of M23C6 particles, formation of sub-grain boundaries and recovery. G91 steel rolled at 550 °C and tempered at 650 °C is found to have best combination of strength and fracture toughness at RT, which may be attributed to large number of small size of precipitates and increase in dislocation density. However, specimen normalized at 1050 °C and tempered at 650 °C shows more than 70% improvement in tensile strength at 650 °C as compared to as-received condition, which shows that G91 steel is more amenable to precipitation hardening. A typical type of tensile fracture with radial cracks and axial splitting is noticed at RT and − 196 °C. The tendency and severity of such radial cracks and axial splitting are found to be affected by stress triaxiality and test temperature.

The precipitation of hydrides normal to the transverse direction in Zr-2.5%Nb pressure tube alloy when cooled under stress above a threshold value, is known as stress reorientation of hydrides. The ex-situ threshold stress values reported in the literature were determined using pre-charged samples heated to a peak temperature to dissolve all the hydrides, cooled to reorientation temperature followed by cooling under stress from reorientation temperature. Leger and Donner performed in-situ reorientation experiments using samples having hydride deposited on its surface. In both cases, hydride reorientation occurred during cooling. In the present investigation, for the first time, experiments were done to study the reorientation phenomenon using in-situ gaseous charging of samples under stress at a constant temperature using a specially designed set up. It is expected that the threshold stress determined by in-situ method used in the present investigation will more closely resemble the formation of radial hydrides under reactor operating conditions. In this work, the threshold stress values for reorientation of hydrides in Zr-2.5%Nb pressure tube material were determined using both in-situ and ex-situ methods.

In the present work, fatigue crack growth (FCG) behavior of modified 9Cr-1Mo steel has been investigated near fatigue threshold and Paris regime at room temperature (RT) and 650 °C. The modified 9Cr-1Mo steel was subjected to normalizing at 1050 °C followed by tempering (650-750 °C) and warm rolling at 550 °C followed by tempering in order to improve its precipitation state and refine microstructure. The microstructural parameters such as grain size, block size, lath width and precipitate size were examined and correlated with FCG and tensile properties of investigated samples. The results showed that decreasing tempering temperature significantly reduces the fatigue threshold at RT while increases the yield strength at elevated temperature (i.e., 650 °C). Increase in fatigue threshold is found to increase with the increase in block size, whereas high density of fine precipitates contributed the high temperature strength. The FCG rate in Paris regime was less affected by heat treatment and rolling process at RT. However, rolled sample tested at 650 °C shows a decrease in FCG rate as compared to heat-treated samples. In terms of higher FCG resistance and yield strength, warm rolling followed by tempering at low temperature (i.e., 700 °C) can be considered optimum. For the numerical simulation of FCG behavior, extended finite element method (XFEM) along with Paris law has been implemented in ABAQUS using python script. The simulated FCG behavior is found in good agreement with the experimental results.

Pressure tube spools fabricated from Zr-2.5%Nb alloy, were subjected to gaseous hydrogen charging and stress reorientation treatment to form radial hydrides in a specially designed fixture. Curved Compact Tension specimens of 17 mm width containing 100 wppm of hydrogen were made using electro discharge machining. Some of the samples were annealed at 300, 325 and 350 °C followed by furnace cooling. Metallographic examination of the samples revealed 100% radial hydride in the samples to which stress reorientation treatment was imparted. The radial hydride fraction decreased with increase in the temperature of annealing that followed stress reorientation. Fracture toughness of samples with variable radial hydride fraction was evaluated as per ASTM E1820-13 procedure between 25 and 300 °C. All the samples exhibited a sharp ductile to brittle transition behavior and the transition temperature was observed to increase with increase in the radial hydride fraction.

Highly dispersed nanoparticles (NPs) of Pd and Pd-Ru alloys on the 10 wt% CoWO4-doped GNS (graphene nano sheets) support have been obtained by a microwave-assisted polyol reduction and investigated for their application as efficient electrode materials for methanol oxidation reaction (MOR). Structural and electrocatalytic surface characterization of hybrid materials were carried out by XRD, TEM, XPS, cyclic voltammetry and chronoamperometry. Pure CoWO4 and CoWO4-doped GNS follow the monoclinic crystal structure and the Pd NPs (6–7 nm) dispersed on CoWO4-doped GNS follow the face-centered cubic crystal structure. It is observed that with the increase of Pd loading from 5 to 20 mg on the support, the onset potential (Eop) for MOR shifts negatively and the MOR current density increases, the magnitude of shift in Eop and increase in the MOR peak current density being the greatest in the case of 15 mg Pd loading. Introduction of Ru from 0.6 to 2.0 mg into 15 mg Pd on the catalyst support, the apparent activity of the active catalyst, 15Pd/10 wt% CoWO4-GNS improved further, the magnitude of improvement, however, being the greatest (≈50%) with 1.0 mg Ru. Thus, novel 15Pd-1.0Ru/10 wt%CoWO4-doped GNS can be a promising electrode material for MOR in alkaline solutions.

This paper presents outcome of round robin exercise on usage of ball indentation technique to predict strength properties of three different materials which are used in Indian nuclear reactors. The main objectives were to evolve a standardized procedure of determining the mechanical properties by the Ball Indentation technique and quantification of the variation in strength and fracture properties due to ageing through such tests. Standard uniaxial tensile tests were also carried out to quantify the differences between strength properties predicted by ball indentation and those determined using conventional tests. Three different materials studied were, carbon-manganese steel (Grade: SA 333Gr.6), stainless steel (SA312 Type 304LN) and zirconium alloy (Zr-2.5Nb). The carbon-manganese steel and stainless steel materials were drawn from extruded pipes while Zr-2.5Nb was drawn from pressure tubes, used in Indian Pressurized Heavy Water Reactors. Ball indentation tests were carried out on materials in as received and in aged conditions. The ageing was simulated artificially. The carbon-manganese steel and stainless steels were subjected to cold work while Zr-2.5Nb alloy was subjected to heat treatment at different temperatures with varying hold times and in some cases with charged hydrogen. The round robin exercise was conducted within different research centers of Department of Atomic Energy (DAE), India. This was a blind exercise in which all the conventional tests were carried out by a group (who was not involved in Ball Indentation tests) after the completion of ball indentation tests by participants. The paper presents the prediction of strength properties, using ball indentation, by different participants. The reasons of scatter in the results among the various participants and differences with respect to conventional test results are discussed in the paper.

Precipitation of δ-hydride in α-Zr matrix deleteriously affects the matrix toughness. Hence, the formation of these hydrides and their equilibrium shape and orientation is studied. The precipitation of hydrides is associated with a volume change and associated chemical free energy. Hence, a multi-physics finite element model is developed to determine the optimal shape and orientation of the hydrides by minimizing accommodation free energy. The model is applied at various temperatures viz., 25, 100, 200 , 300 and 400 °C. Three cases of fully elastic, elastic perfectly-plastic and elastic work-hardening plastic systems are studied and compared. The model takes into account the temperature variation of stiffness tensor, yield strength, hardening coefficients, volumetric change in the precipitation process and volume change in matrix due to hydrogen depletion. It is found that a flat hydride with its geometric orientation such that its major axis coincide the basal plane of the parent matrix is the most optimal shape and orientation. This is irrespective of whether the system is modeled as fully elastic, elastic perfectly-plastic or elastic work-hardening plastic. The results are discussed based on microscopic experiments and from experimental observations from the literature.

The cleavage fracture probability of ferritic and ferritic/martensitic steels is generally computed using Beremin's model. In this work, a novel approach is developed for Beremin's model to be applicable to crack tip conditions under nonself similar stress field. The modification of Beremin's model corrects the inaccurate integration of local cleavage fracture probabilities of reference volume elements due to the incorrect assumption of self similar stress field under large-scale yielding. Under small-scale yielding condition, where stresses remain self similar, integration of local cleavage failure probability is accurate. However, when the stresses differ along the thickness due to constraint loss, the integration of local cleavage fracture probability is inaccurate due to the fact that the stress levels are not same in all the reference volume elements. The modification developed in this work in the form of a parameter named Weibull Triaxiality, which measures the deviation of a crack tip volume from one under self similar stress field.

Highly dispersed nanoparticles (NPs) of Cu, Ag and their composites are obtained on nitrogen-doped graphene nanosheets (N-GNS) using a microwave-assisted polyol reduction method. Structural, electrochemical and electrocatalytic surface characterization of materials have been carried out by TEM, XRD, XPS, cyclic (CV) and linear sweep (LSV) voltammetries. The study shows that the crystallites of the Cu-Ag composite follow the face-centered cubic (fcc) crystal structure. Further, the catalytic activities of Cu/N-GNS and Ag/N-GNS towards oxygen reduction reaction (ORR) are greatly enhanced when Cu and Ag are dispersed on the N-GNS surface in 1:2.9 M ratio. The active composite material does not oxidize methanol practically and has outstanding stability under ORR condition.

Axial fracture toughness of cold worked and stress relieved Zr-2.5Nb pressure tube material containing deuterium between 5 and 200wppm was determined as per ASTM E−1820-15 between 25 and 300 °C using 17 mm wide Curved Compact Tension samples. For deuterium charged materials, the variation of the fracture toughness with test temperature showed typical S curve behavior. At 25 °C, the fracture toughness dropped drastically between 5 and 30wppm of deuterium concentration and afterwards it was constant between 30 and 200wppm. Samples having similar axial split density had similar fracture toughness. The lower-shelf and upper-shelf temperature was attributed to a critical axial split density and disappearance of axial split on fracture surface, respectively.

An experimental setup was designed, fabricated and used to form radial hydrides in Zr-2.5%Nb alloy pressure tube spool. The design of setup was based on ensuring a hoop stress in the spool greater than threshold stress for reorientation of hydrides in this alloy, which was achieved by manipulating the thermal expansion coefficient of the plunger and pressure tube material and diametral interference between them. The experimental setup was loaded on a universal testing machine (UTM) fitted with an environmental chamber and subjected to a temperature cycle for the stress reorientation treatment. The metallographic examination of the hydrogen charged spools subjected to stress re-orientation treatment using this set up revealed formation of predominantly radial hydrides. The variation of fracture toughness of material containing radial hydride with test temperature showed typical 'S' curve behavior with transition temperatures more than that of the material containing circumferential hydride. (C) 2017 Elsevier B.V. All rights reserved.

Mechanical properties of nanocrystalline (nc) Nickel (Ni) and Ni-Teflon (Ni-Tf) nanocomposite has been studied in detail using nanoindentation and conventional tensile tests. Nanoindentation tests carried out at room temperature over a range of loading rate exhibited higher hardness for the nanocomposite as compared to the nc-Ni due to Tf dispersion. An increase in strain rate sensitivity (SRS) and decrease in activation volume of the nanocomposite as compared to nc-Ni was also observed. Tensile tests were carried out over a temperature regime of room temperature (RT) to 500 °C. Tensile data also showed enhancement of yield stress and ultimate tensile strength up to 150 °C. But from 200 °C and above, Tf dispersion is unable to provide superior strength over nc-Ni as both the materials show almost similar strength. However, the ductility of the Ni-Tf nanocomposite was observed to be higher at all temperature compared to the pure nc-Ni. SRS was found to increase with temperature while SRS in nanocomposite had higher value below 200 °C. The present study reveals that Ni-Tf nanocomposite possess better strength and ductility compared to nc-Ni up to 200 °C.

Hydrogen diffuses in the zircaloy-4 fuel clad tube during its service and precipitates as brittle hydride phases beyond terminal solid solubility of precipitation. Brittle hydride precipitates in the region of high stress like crack tips and assists crack propagation leading to failure of the cladding. In order to elucidate the effects of hydride precipitated at the tip of the crack on crack propagation behaviour in zircaloy-4, the numerical simulation is performed on compact tension specimens using the extended finite element method (XFEM) in Abaqus 6.12. An attempt has been made to understand the effects of crack and hydride dimensions on crack propagation by evaluating stress intensity factor and J-integral for different length of crack and hydride. Their values indicate that hydride assists the crack propagation, however, if the length of crack exceeds certain critical length, the effect of hydride precipitation on crack propagation is minimal. The values of stress intensity factor and J-integral are also compared with the experimental values reported in the literature and found to be concordant.

Modified 9Cr-1Mo steel is a candidate material for high-temperature (~650 °C) application in the development of Generation-IV nuclear reactors. In the present investigation, intermediate hot rolling on modified 9Cr-1Mo steel have been performed in austenitic phase (1050 °C) and below austenitic phase (550 °C). Intermediate hot rolling including normalization at elevated temperature (1150 °C), followed by tempering (700 °C) is employed to refine its lath structure and improve precipitation state. The modified 9Cr-1Mo steel rolled at 1050 °C with 30% deformation showed improvement on both strength and toughness, which may be attributed to increase in area fraction of M23C6/MX precipitates and higher dislocation density. A typical mode of tensile fracture with radial cracks and axial splitting is observed in the testing temperature between −196 and 20 °C. These cracks are caused by a high fracture stress generated in perpendicular direction during necking since stress triaxiality increases as necking is deepened. However, such radial cracks are found completely absent in the test temperature between 100 and 650 °C.

Delayed Hydride Cracking (DHC) behaviour of Zr-2.5Nb Pressure Tube (PT) material used in Indian 200MWe Pressurised Heavy Water Reactor (PHWR) containing radial hydrides and circumferential hydrides was studied in the temperature range of 200–250 °C. A section of the PT was charged with 70wppm of hydrogen and successfully subjected to re-orientation treatment to form radial hydride. The radial hydride in the reoriented tube was characterised by the hydrogen continuity co-efficient (HCC). It was observed that the average DHC velocity (VDHC) for the radial hydrided sample was 2.5 and 1.85 times faster than the circumferential hydrided sample at 200 and 225 °C, respectively. The VDHC at the temperature of 250 °C, for both the radial hydrided and the circumferential hydrided sample were comparable. The average VDHC for circumferential hydrided sample varies linearly with 1/T while it was observed to vary non-linearly with 1/T for radial hydrided sample. The VDHC of the radial hydrided sample was observed to increase with increase in the stress intensity factor, whereas for circumferential hydrided samples the VDHC was independent of stress intensity factor in the range of 18–45MPa.m1/2.

A burst criterion model accounting the simultaneous phenomena of corrosion, solute-strengthening effect of oxygen, oxygen concentration based non-isothermal phase transformation, and thermal creep has been developed to predict the rupture behaviour of zircaloy-4 nuclear fuel cladding during the loss-of-coolant accident transients. The present burst criterion model has been validated using experimental data obtained from single-rod transient burst tests performed in steam environment. The predictions are in good agreement with the experimental results. A detailed computational analysis has been performed to assess the role of different parameters in the rupture of zircaloy cladding during loss-of-coolant accidents. This model reveals that at low temperatures, lower heating rates produce higher burst strains as oxidation effect is nominal. For high temperatures, the lower heating rates produce less burst strains, whereas higher heating rates yield greater burst strains.

Tensile properties of autoclaved Zr-2.5Nb pressure tube material containing hydrogen isotope between 5 and 200 wppm were evaluated between 25 and 300 °C using specimens with its axis oriented along longitudinal direction of the tube. Analysis of tensile test results showed that both YS and UTS of this alloy decreased linearly with increasing test temperature. The uniform and total plastic strain decreased marginally with increase in test temperature. At all test temperatures, before necking tensile properties were unaffected by hydrogen isotope concentration whereas hydrogen isotope had clear effect on post-necking tensile properties especially at 25 and 100 °C. Post-necking ductility showed a transition behavior at 25 and 100 °C and it was able to capture the effect of hydride embrittlement in this material.

Fatigue crack growth (fcg) behavior of cold-worked and stress relieved Zr–2.5 Nb was studied in the longitudinal (with and without hydrides) and transverse direction at ambient temperature and load ratio of 0.1 using compact tension samples. Fatigue loading in the transverse direction (distribution of both hard and soft grains) showed facet formation on the fracture surface and the highest Δ K th whereas loading in the longitudinal direction (distribution of primarily soft grains) showed no facet formation and a lower Δ K th . Hydrided Zr–2.5 Nb loaded in the transverse direction showed large facets with the lowest Δ K th . Texture influenced fcg at low Δ K but not at higher Δ K .

The fracture behaviour of ferritic and ferritic/martensitic steels in Ductile to Brittle Transition (DBT) region is precisely captured by Master Curve (MC) approach, except in the upper region of transition where the MC is not applicable due to excessive prior ductile tearing. The MC approach necessarily models cleavage and assumes self-similarity of stress field at the crack front. This single fracture parameter, K JC , based approach cannot be applied to upper region of DBT where significant amount of ductile tearing precedes cleavage. The reason for its inapplicability in upper region of DBT is the increasing active volume which can be sub-divided into a part changing to plastic wake after ductile tearing and other contributing to cleavage. The part of volume forming plastic wake is not taken into consideration by MC approach. Furthermore, the increment in constraint with associated change in ligament length due to ductile tearing increases the number of potentially critical carbides for cleavage succeeding eventually in cleavage fracture. In the conventional MC approach the fracture data associated either with constraint loss or DCG prior to cleavage are censored with the limiting values described in ASTM E1921-14a. This censoring increases the number of tests required for determination of reference transition temperature, T 0 . Moreover, the fracture behaviour of upper region of transition is not captured by conventional MC approach. Utilizing the fact that any fracture test ending by cleavage contributes to probability of cleavage fracture, a constraint based modification is applied to the conventional MC approach. The Ductile Crack Growth (DCG) in the upper region of DBT, increased sampled volume associated with the ductile tearing and constraint increment are addressed in this work using a function of T stress . This single function has been found to incorporate the interrelated phenomena of constraint and sampled volume increment associated with the probability of cleavage fracture with prior ductile crack growth in upper region of DBT. This modified approach is investigated in this study using GTN local ductile damage to simulate DCG in upper region of DBT. The modified MC is applied on Euro fracture dataset which consists of specimens fractured in tension (CT specimens of 4T, 2T, 1T and 0.5T thickness) as well as on both tension and bend specimen dataset generated on newly developed Indian reduced activation Ferritic/Martensitic steel. It was found that the modified MC is applicable to test data in greater range of transition in comparison to conventional MC. Additionally, this modified MC estimates T 0 close to one estimated by conventional MC by using dataset which otherwise would be censored in conventional MC approach.

The assessment of fracture behaviour of ferritic/ferritic martensitic steels in Ductile to Brittle Transition (DBT) region iscrucial for shallower cracks. In this work the fracture behavior of In-RAFM steels isinvestigated with systematically varying a/W using Master Curve approach. This a/W effect is investigated using σ∗-V∗approach by calculating the active volume at simulated crack front. It is observed that the fitting parameter of σ∗-V∗approach is in direct correlation with changing constraint parameter ΔT stress . The constraint effect with varying crack depth is also captured by a novel constraint parameter referred as Weibull triaxiality.

Zr-2.5Nb alloy pressure tubes used in pressurized heavy water reactors (PHWR) are susceptible to failure by Delayed Hydride Cracking (DHC), which is a form of localized hydride-embrittlement phenomenon manifested in the presence of a hydrostatic stress gradient and hydrogen concentration above a threshold limit as a sub-critical crack growth process in hydride forming metals. The DHC parameters used for safety assessment are DHC velocity (VDHC) and a threshold stress intensity factor (KIH). In this work DHC velocity was determined for Zr-2.5Nb pressure tube material at 250 and 280 °C using specimens of thickness between 1 mm and 4.5 mm. The DHC velocity was found to increase with increase in specimen thickness at 250 °C and 280 °C. Significant amount of tunneling of the crack was observed for 1-mm and 2-mm thick specimens at 250 °C and 280 °C, with the degree of tunneling increasing with decrease in thickness and increase in test temperature.

In order to get an idea of the statistical variation in the tensile properties of the double-melted as well as quadruple melted Zr-2.5 Nb pressure tubes (PTs) and also the variation in tensile properties between the two ends of the pressure tubes, tension tests were carried out on around 50 pressure tube off-cuts. Longitudinal and transverse tensile specimens were prepared from these off-cuts of pressure tubes of double-melted and quadruple melted types. For quadruple melted pressure tubes the specimens were tested from both front-end and back-end off-cuts. Miniature flat tensile specimens having 1.8 mm width and 1.5 mm thickness and 7.6 mm gauge length were prepared from the pressure tube off-cuts without any flattening treatment. Tension tests were carried out in a screw-driven machine at room temperature and 300 °C for both front-end and back-end off-cuts of each of 16 pressure tubes. In general the transverse specimens showed higher yield strength (YS) and ultimate tensile strength (UTS) compared to the longitudinal specimens. Transverse specimens showed less strain hardening compared to the longitudinal specimens. The axial specimens showed higher uniform (UE) and total elongation (TE) compared to the transverse specimens. Double-melted pressure tubes showed relatively higher strength and lower elongation and larger standard deviation compared to the quadruple melted pressure tubes. Mean values of tensile properties showed that back-end off-cuts were relatively stronger and less ductile compared to the front-end off-cuts.

Cyclic voltammetry (CV) for Pd electrodeposition on Au from PdCl2+HCl solution between +1.100 and –0.400 V vs Ag/AgCl (3M NaCl) was performed. The CVs exhibit nucleation loop (NL), which can be characterized by higher cathodic current density (j) in the reverse scan rather than that in the forward scan. The reasons for the appearance of NL are investigated by making use of the surface condition of electrode and overpotential (η). Since it is difficult to estimate η during CV, it was obtained by potentiostatic electrodepositions (ED) on the substrates with similar surface conditions as those within NL. These substrates were produced by a potential sweep in (i) forward (FS) and (ii) forward+reverse (RS) scans from open circuit potential up to a representative potential (+0.330 V) within NL. These FS and RS substrates possess partial and significant Pd coverage, respectively. ED at +0.330 V on these substrates shows similar j trends in the initial stages as in NL. η during ED is estimated from the equilibrium potentials and the potential drop due to solution resistance (Rs). Rs is obtained from electrochemical impedance spectroscopy at +0.330 V. The estimated η is higher on RS, supporting higher j during electrodeposition on RS and reverse scan of CV within NL. Electrochemical and morphological analyses suggest that Pd deposition on the FS is nucleation driven whereas that on the RS is growth driven with the surface condition playing an important role in the appearance of NL during CV.

The crack growth rates of un-irradiated and un-hydrided Zr-2.5Nb pressure tube material has been investigated in both air and in vacuum. Pre-cracked CCT samples made from pressure tube were tested in air atmosphere at 300 °C and 350 °C at different stress intensity factors. It was observed that till a particular stress intensity factor there is no crack growth and after which the crack growth rate increases with increase in stress intensity factor. For the test carried out in vacuum, it was observed that the crack growth rates for air is an order of magnitude higher than that of the crack growth obtained in the vacuum ambience.

In the present study, influence of conventional normalizing (1050 °C) and double austenitization treatment (950–1050 °C) followed by tempering (750 °C) have been investigated to study the fatigue threshold, tensile strength and high cycle fatigue behavior of modified 9Cr–1Mo steel at room temperature (24 °C). By designing a double austenitization treatment, a homogeneous tempered martensitic microstructure with improved area fraction of precipitates and small prior austenite grain size can be obtained. As a result, improvement in fatigue threshold and tensile strength is achieved. However, a small decrease in fatigue strength is observed. Fatigue strength is affected due to presence of coarse particles precipitated along the grain boundaries. Fine particles precipitated along the grain boundaries create more barriers for the dislocation motion. As a result, more cycles are required for crack initiation. A damage model based on continuum damage mechanics is coupled with extended finite element method to obtain the fatigue life. The fatigue life predicted using numerical simulation found in good agreement with the experimental results. This study shows the effectiveness of numerical simulations to perform fatigue crack growth analysis using continuum damage mechanics and extended finite element method. Fractographs of tensile sample shows the presence of cracks in radial direction and axial splitting. However, such unique behavior is not obtained in high cycle fatigue samples.

Pd is an excellent catalyst layer for Mg-based thin films for hydrogen storage. Cyclic voltammetry (CV) for Pd deposition on Au in PdCl2+HCl solution between +1.100 and –0.400 V vs Ag/AgCl (3M NaCl) exhibits nucleation loop (NL). NL is characterized by higher cathodic current density (j) in reverse scan versus that in forward scan. This work addresses the question “Why does nucleation loop occur during CV?” using substrate conditions and overpotential (η). Since it is difficult to estimate η during CV, the substrate conditions similar to those within NL are reproduced by potential sweep in (i) forward and (ii) complete forward+reverse scans up to a representative potential (+0.330 V) within NL. These scans produce substrates FS and RS with partial and significant Pd coverage, respectively. Eventual potentiostatic electrodepositions (ED) at +0.330 V on these substrates initially show similar j trends as in NL, replicating Pd deposition during CV. η during ED is estimated by equilibrium potentials and potential drop due to solution resistance (R s) from electrochemical impedance spectroscopy at +0.330 V. The estimated η is higher on RS, supporting higher j during electrodeposition on RS and reverse scan of CV within NL. Electrochemical and morphological analyses together suggest that Pd deposition on FS is nucleation driven; that on RS is growth driven, which help in revealing the metal deposition mechanisms.

For fracture testing of pressure tubes, specimens are machined directly from the pressure tube without disturbing its curvature. The compact tension (CT) specimens thus obtained are called curved compact tension (CCT) specimens. Apart from being curved, the CCT specimens do not satisfy width/thickness criteria as mentioned in ASTM E1820-15. In spite of these differences, CCT specimens use the same ηpl and χpl factors as that of standard CT specimens for fracture toughness evaluation. In this work, 17 mm wide blunt notched CT specimens of different crack length obtained from a Zr-2.5Nb pressure tube, which were not meeting the thickness criteria, were used to investigate load separation and to evaluate the ηpl factor experimentally. Load separation existed between a/W of 0.45 and 0.70. The ηpl factor had a constant value of 2.078 in this range when calculated using the load separation method by power law fit. The ηpl factor calculated using a third order polynomial fit passing through the origin showed linear increase with b/W similar to but marginally lower than standard CT specimen.

Silicon carbide (SiC) particles were dispersed in nanocrystalline Ni (nc-Ni) matrix and thermal grain growth was studied. Microstructural characterization revealed that while the average grain size of as prepared foils of both nc-Ni and Ni-SiC nanocomposite was found to be ∼ 60 nm, presence of SiC particles restricted thermal grain growth remarkably. While the grain size of nc-Ni annealed at 300°C was found to be 210 nm that of Ni-SiC nanocomposite was only ∼100 nm. Differential Scanning Calorimetry (DSC) results exhibited higher heat flow in case of nc-Ni as compared to the Ni-SiC nanocomposite which signifies rapid thermal grain growth in the former one. Tensile tests carried out on both the materials showed that yield strength (YS) of the nanocomposite was always higher than nc-Ni. However, the ultimate tensile strength (UTS) of the former one was lower up to 100°C which might be due to premature brittle fracture initiated along SiC particle interface. At 200°C and above both YS and UTS were found to be higher for nanocomposite due to easier plastic flow being enabled at elevated temperature.

In order to attain improved in-reactor performance few prototypes pressure tubes of Zr-2.5Nb alloy were manufactured by employing forging to break the cast structure and to obtain more homogeneous microstructure. Both double forging and single forging were employed. The forged material was further processed by employing hot extrusion, cold pilgering and autoclaving. A detailed characterization in terms of mechanical properties and microstructure of the prototype tubes were carried for qualifying it for intended use as pressure tubes in PHWR700 reactors. In this work, Delayed Hydride Cracking (DHC) behavior of the forged Zr-2.5Nb pressure tube material characterized in terms of DHC velocity and threshold stress intensity factor associated with DHC (KIH) was compared with that of conventionally manufactured material in the temperature range of 200–283 °C. Activation energy associated with the DHC in this alloy was found to be ∼60 kJ/mol for the forged materials.

The pressure tubes of Indian Pressurized Heavy Water Reactor (IPHWR) of 220 MWe are made of Zr–2.5Nb alloy manufactured either from Double Melted (DM) or from Quadruple Melted (QM) ingots. These pressure tubes are manufactured by hot extrusion, two stages of cold pilgering with intermediate annealing and autoclaving. To achieve good in-reactor performance, it is required to have minimum variability in the mechanical properties of the pressure tube across its length and between tube to tube. In this work, tensile properties and fracture toughness parameters (Jmax, dJ/da and CCL determined as per ASTM E1820-11 standard) of unirradiated Zr–2.5Nb alloy pressure tubes manufactured from DM and QM ingots using samples obtained from front and back end of the tubes is presented. The mechanical properties were evaluated in temperature range of 25–450 °C and compared with the corresponding data reported in literature for CANDU pressure tubes.

The role of pre-strain on hydrogen embrittlement susceptibility of Indian Reduced Activation Ferritic Martensitic Steel was investigated using constant nominal strain-rate tension test. The samples were pre-strained to different levels of plastic strain and their mechanical behavior and mode of fracture under the influence of hydrogen was studied. The effect of plastic pre-strain in the range of 0.5–2% on the ductility of the samples was prominent. Compared to samples without any pre-straining, effect of hydrogen was more pronounced on pre-strained samples. Prior deformation reduced the material ductility under the influence of hydrogen. Up to 35% reduction in the total strain was observed under the influence of hydrogen in pre-strained samples. Hydrogen charging resulted in increased occurrence of brittle zones on the fracture surface. Hydrogen Enhanced Decohesion (HEDE) was found to be the dominant mechanism of fracture.

Fracture toughness of irradiated Zr–2.5Nb alloy pressure tube, fabricated by the cold pilgering and stress relieving route, was evaluated using disk compact tension type specimens. These specimens were punched out from the irradiated pressure tube (S-07), which was in service for about 8 effective full power years of reactor operation in the Kakrapar Atomic Power Station-2 (KAPS-2). The tests were carried out remotely inside a lead shielded enclosure. Crack growth during the test was measured using the direct current potential drop technique. The irradiated pressure tube showed low fracture toughness at 25 °C. The fracture toughness increased with increase in temperature up to 250 °C but was practically unaffected with further increase in temperature up to 300 °C. This paper discusses the fracture behavior of irradiated Indian pressure tube material and compares it with other data available.

Binary nano-composites of palladium and a metal (Fe or Cu) manganite on graphene nanosheets (GNS) have been prepared by a microwave-assisted polyol reduction method and investigated as electrocatalysts for the methanol oxidation reaction (MOR) in 1 M KOH at 25 °C. Structural and electrocatalytic surface characterizations of composites are carried out by X-ray diffraction, transmission electron microscopy, X-ray photoelectron microscopy, cyclic voltammetry and chronoamperometry. Results show that new composite catalysts, particularly 40 wt%Pd–x wt%FeMn2O4/GNS (where x = 5, 8, 10 & 15) are MOR active and that the activity is the greatest with the catalyst containing 8 wt% of the oxide. The composite, Pd–8 wt%FeMn2O4/GNS, exhibits much superior catalytic activity as well as stability compared to the base (Pd/GNS) electrode. The enhanced catalytic activity and stability of the Pd/GNS catalyst in presence of the oxide can be ascribed to increased population of adsorbed OH− ions/OH radicals at the electrode surface.

Impact toughness of Zr-2.5Nb pressure tube material was evaluated as a function of specimen geometry, orientation and test temperature. Two types of samples were used. Curved impact specimens were machined directly from the tube section thereby retaining the pressure tube curvature and flat impact specimens were machined from a flattened tube section subjected to stress relieving treatment. In both the cases the specimen thickness was the actual pressure tube thickness that is 3.5 mm and the other dimension of the impact specimen was 55 mm × 10 mm. The tests were carried out for both as-received and hydrided pressure tubes between room temperature and 300 °C. As the Zr-2.5Nb pressure tube material is anisotropic, the studies were also carried out using specimens with crack growth either along axial or along transverse direction of the tube. The paper describes the results generated and discusses the materials impact toughness behaviour under different conditions.

It is a common belief that embedded expanding inclusions are subjected to an internal homogeneous compressive hydrostatic stress. Still, cracks that appear in precipitates that occupy a larger volume than the original material, are frequently observed. The appearance of cracks has since long been regarded as a paradox. In the present study it is shown that matrix materials that increases its volume even several percent during the precipitation process develop a tensile hydrostatic stress in the centre of the precipitate. This is the result of a complicated mechanical-chemical phase transformation process. The process is here studied using a Landau phase feld model. Before the material is transformed and incorporated in a precipitate it undergoes stretching beyond the elastic strain limit because of the presence of already expanded material. During the phase transformation, the accompanying volumetric expansion cannot be fully accommodated which instead creates an internal compressive stress and adds tension in the surrounding material. As the growth of the precipitate proceeds, a region with increasing tensile stress develops in the interior of the precipitate. This is suggested to be the most probable cause of the observed cracks. First the mechanics that lead to the tension is computed. The infuence of elastic-plastic properties is studied both for cases both with and without cracks. The growth history from microscopic to macroscopic precipitates is followed and the result is compared with observations of so called hydride blisters that are formed on surfaces of zirconium alloys in the presence of hydrogen. A common practical situation is when the zirconium is in contact with an object of lower temperature. Then the cooled spot attracts hydrogen that make the zirconium transform to a metal hydride with the shape of a blister. The simulations predicts a final size and position of the growing crack that compares well with the experimental observations. open access

The deformation and fracture behaviour of AISI 403, a tempered martensitic stainless steel for end fitting application of Pressurised heavy water reactor is being reported. The deformation behaviour studies entailed characterisation of tensile behaviour in the temperature range 77 - 873 K for the as recieved and the fine grained Nb modified variant of AISI 403. the study of elevated tensile behaviour in the two steels has been undertaken with the purpose of characterising the strain rate - temperature domain of the occurrence of dynamic strain aging (DSA) phenomenon. In both steels, while the temperature range for the manifestations of characteristic anomalies in the tensile curve due to DSA was observed within 523 - 673K, the strain domain for the fine grained Nb modfied variety was significantly higher as comapred with the as recievied variety. The low temperature tensile tests for the as recieved AISI 403 revealed the presence of Pseudo=alloy softening in the temperature range 273 - 193 K. The effect of high DBTT of the AISI 403 steel was shown by the fracture toughness tests in the J-integral format at room temperature that displayed significant scatter in smaples with high in-plane and out of plane constraint. Smaples with lower constraint showing stable crack growth were further tested at high temperature to obtain the temperature dependence of initiation fracture toughness and propagation touhgness. Within the DSA tempertaures a sharp decline in the fracture properties were observed. A mechanistic interpretation for the manifestations of the various observed phenomena is presented.

The stress fields of expanding (precipitation) and contracting (dissolution) hydride plates were computed by finite element method using Zr-H solid solution and hydride properties at 25, 200 and 400 degrees C for fully and semi-constrained hydride plates. For the first time simultaneous hydride expansion and matrix contraction and vice-versa have been considered in a simulation of hydride precipitation and dissolution, respectively. It was observed that a fully constrained expanding hydride plate exerts a tensile stress field in the matrix close to the edge of the hydride plate while a partially contracting hydride plate exerts a tensile stress field in the hydride plate as well as a large compressive stress in the surrounding matrix close to the edge of the hydride plate. It is suggested that a compressive stress component in the matrix acting normal to a partially shrinking hydride plate could possibly explain an enhanced resistance to hydride embrittlement of Zr-alloy at elevated temperature. (C) 2013 Elsevier B.V. All rights reserved. (Less)

The fracture toughness of Zr–2.5Nb pressure tube alloy used in Indian Pressurized Heavy Water Reactor was determined in the temperature range of 30–300 °C using load normalization technique ( LNT ) and a comparison between the crack initiation and the crack propagation toughness parameters is presented with the values obtained using direct current potential drop ( DCPD ) technique. The dependence of error in J -parameters determined using LNT to that obtained using DCPD on temperature and final crack extensions was also investigated. Below 200 °C, the error exhibited weak dependence on final crack extensions. Above 200 °C the error in initiation toughness increased significantly while the error in propagation toughness was practically unaffected.

In this work, influence of hydrogen and temperature on the fracture toughness parameters of unirradiated, cold worked and stress relieved (CWSR) Zr-2.5Nb pressure tube alloys used in Indian Pressurized Heavy Water Reactor is reported. The fracture toughness tests were carried out using 17 mm width curved compact tension specimens machined from gaseously hydrogen charged tube-sections. Metallography of the samples revealed that hydrides were predominantly oriented along axial-circumferential plane of the tube. Fracture toughness tests were carried out in the temperature range of 30-300 degrees C as per ASTM standard E-1820-06, with the crack length measured using direct current potential drop (DCPD) technique. The fracture toughness parameters (J(Q), J(Max) and dJ/da), were determined. The critical crack length (CCL) for catastrophic failure was determined using a numerical method. It was observed that for a given test temperature, the fracture toughness parameters representing crack initiation (J(Q)) and crack propagation (J(Max), and dJ/da) is practically unaffected by hydrogen content. Also, for given hydrogen content, all the aforementioned fracture toughness parameters increased with temperature to a saturation value. (c) 2012 Elsevier B.V. All rights reserved. (Less)

Zirconium alloy tubes act as miniature pressure vessels in Pressurized Heavy Water Reactors and are manufactured following a series of thermo-mechanical processing steps under optimized conditions which imparts the desirable microstructure and initial mechanical properties to the components under the most demanding service conditions inside a nuclear reactor. The service environment (temperature, stress, corrosion, and irradiation) leads to changes in dimensions (creep and growth); degradation in mechanical properties; modification in microstructure and to pick up of hydrogen. This paper summarizes various issues related to structural integrity of this important component with emphasis on deformation behavior during processing and service and hydrogen related embritllement.